Vertical angles are congruent, so you can set Angle A = Angle C and solve for x.
Angle A = Angle C
4x+12 = x+18 Get x terms on L side of = & constants (plain #) on R side by subtracting x on both sides.
3x+12= 18 Now, subtract 12 from both sides of equation to get constants on R side of equation.
3x = 6 Now, divide by 3 to solve for x.
x = 2